Within Advanced Television Standards Committee (ATSC) compliant video receivers, trained channel estimation is performed utilizing the field sync available in the ATSC field. However, one trained equalization per field is not sufficiently frequent to estimate even slowly varying channels. Blind equalization techniques applied to intra-field channel estimation have not been entirely successful on all important channels. Other proposals to improve the trained equalization update rate require a new training signal and are therefore not backward compatible with existing systems, and also improve channel estimation at the cost of useful data rate.
One alternative to existing equalization schemes which might improve channel estimation without degrading the data rate is frequency domain equalization. However, current proposals for frequency domain equalization for monocarrier systems such as vestigial sideband (VSB) modulation are essentially time-domain equalization performed in the frequency domain. Coefficients, updates, and finite impulse response (FIR) tap calculations are performed in the frequency domain with the assistance of a fast Fourier transform (FFT) of the incoming data. Since the training sequence is defined only in the time domain, the error calculation must be performed on the filtered data after retransformation to the time domain. The error is then retransformed to the frequency domain for coefficient update. Similar restrictions apply to statistical (blind) and decision directed error calculations.
There is, therefore, a need in the art for improving the training signal available for channel estimation. It would be desirable to provide an enhancement to the training signal without degrading data rate and allowing existing receivers to continue to operate as intended.